The research is concerned with two instances of post-transcriptional control of translation of mRNA in embryonic systems. The first is an interordinal sand-dollar x sea urchin hybrid embryo which synthesizes only maternal proteins, except for histones. The polysomes of the blocked hybrid embryos contain RNAs of both parentages equally, but the paternal set is not being measurably translated. An immunological and electrophoretic study will be extended to determine both the totality of the translational block and the relationship of its onset to the morphological abnormalities which appear. To determine if both genomes are represented as completely as in normal embryos, the hybrid polysomal mRNAs have been examined by molecular hybridization analysis in DNA excess reactions, and their kinetic complexity determined. Three studies of the mechanisms of the translational block are planned: the rate of translation of the hybrid and normal polysomes; the presence or absence of paternal nascent peptides on the hybrid polysomes using L125 -anti-sea urchin globulins; and the ability of the hybrid paternal mRNAs to be translated into sea urchin peptides when injected into frog cocytes. These studies of cell hybrids are aimed at examining translational regulation of sets of messages during organogenesis. Translationally arrested informational RNA in unfertilized sea urchin eggs will be studied for its kinetic complexity, its minimal informational content by translation in activated enucleate merogones, and for its molecular relationship to hnRNA and polysomal mRNA by measuring interspersion of reiterated and unique DNA sequence transcripts. Determining the mechanisms of storage and activation of this RNA is significant for understanding egg activation at fertilization and programming of the early embryo. Isolation of 16-cell stage micromeres and of primary mesenchyme cells in various stages of differentiation has been achieved and the translational capacity of these cells is being compared with other gastula cells, in relation to early and late mRNA transcription.